Webformed cross-laminates consisting of two or more mutually bonded films, which are unuaxially oriented or unbalanced biaxially oriented, have been commercially available since 1968, but have always presented problems in heatsealing.
These first cross-laminates have been produced with the compositions, structure and method described in GB-A-0,792,976 of Apr. 9, 1958 and GB-A-0,816,607 of Jul. 15, 1959. A tubular film of polyethylene (normally HDPE) or polypropylene has been strongly uniaxially oriented in its longitudinal direction by stretching at a relatively low temperature, then helically cut to a web with its orientation on bias, and two or more such webs have then been laminated, usually by extrusion lamination, with criss-crossing directions of orientation. When such cross-laminates are heatsealed, e.g. by impulse sealing, to form a xe2x80x9cpeel-typexe2x80x9d heatseal, the impact peel strength of the seal has turned out to be exceptionally low (except if the seal is improved by additional material, e.g. by overtaping).
By xe2x80x9cpeel-typexe2x80x9d heatseal is understood a heatseal adapted to withstand peeling forces concentrated in a lineary very narrow zone, as opposed to an xe2x80x9coverlap typexe2x80x9d heatseal made to withstand the shearing forces, which are distributed over the entire width of the seal.
Impact- or shock-peel strength is particularly needed in heatsealed bags to withstand the shock action when the bag is dropped on one of its edges perpendicular to the linearly extending heatseal.
The poor peeling strength of the seal is due to the fact that the heatsealing ruins the orientation, not only in the bonded part of the sealxe2x80x94where loss of orientation does not matterxe2x80x94but also in immediately adjacent narrow xe2x80x9clinesxe2x80x9d of unbonded cross-laminate. Upon shock-peeling these xe2x80x9clinesxe2x80x9d appear weak and tend to rupture. (At lower velocities of peeling, when the material in these lines has time to elongate and orient, the seals may give a satisfactory peel-strength).
The shock-peel strength can be improved, but not sufficiently, by coextruding thin lower melting surface layers (xe2x80x9cheatseal layersxe2x80x9d) on the films used for manufacture of the cross-laminate. However, when the heatsealing takes place at a temperature at which the orientation in the main layers is maintained, another phenomenon occurs, namely that the two cross-laminates separate from each other since the peeling force is concentrated in the thin bonding layers. To avoid such separation, the films in and immediately adjacent to the bonded zone must be allowed to shrink and thereby grow in thickness, by which the tensions created by the peeling will be reduced.
As described in GB-A-1,526,722 improvements in shock-peel-strength of heatseals (and certain other strength advantages) has been obtained by a different method of producing cross-laminates. In the usual way of practising this method (which has been commercial since 1978) there is first coextruded a tubular film with a generally uniaxial, longitudinal orientation, and with minor surface layers for facilitating the subsequent lamination process (xe2x80x9clamination layersxe2x80x9d), and for improving the surface characteristics of the final cross-laminate, especially its heatsealing properties (xe2x80x9cheatseal layersxe2x80x9d). This tubular film is then helically cut to form a web with melt orientation on bias, and several such webs are continuously cross-xe2x80x9csandwichedxe2x80x9d and further oriented by transverse stretching between grooved rollers andxe2x80x94before or after this operationxe2x80x94subjected to conventional longitudinal stretching between ordinary smooth rollers which rotate at different circumferential velocities. The grooved rollers stretching is usually repeated several times, and the longitudinal stretching may also be repeated. For obtaining maximum energy-absorbing properties, also with a view to the shock-peel strength of the heatseal, the longitudinal and transverse stretching steps after bringing the helically cut films together to a xe2x80x9csandwichxe2x80x9d is normally carried out at temperatures very much below the melting ranges of the films, and may even be carried out at normal ambient temperature. U.S. Pat. No. 4,629,525 discloses a subsequent heat-stabilization process. I have also suggested arranging the films of such laminates such that one has a main direction of orientation generally following the longitudinal direction of the cross-laminate and the other has a main direction of orientation substantially perpendicular to the longitudinal direction.
There is normally achieved a significant bonding between the cross-xe2x80x9csandwichedxe2x80x9d films, so that they will form a laminate, already by the stretching between grooved rollers, and this bonding is increased by the heat-stabilization process. However, a really strong overall bonding is always avoided since this will ruin the tear-propagation resistance, which is a particularly remarkable feature of these cross-laminates.
Improved evenness of the transverse stretching between grooved rollers, and possibilities for higher throughputs, heavier and stiffer cross-laminates is achieved by a method disclosed in EP-B-0,276,100, in which a set of grooved rollers fit extremely fine to each other and act under a high rollers pressure to perform a combined transverse pulling and transverse squeezing. (The example in this present specification makes use of this). The said method is further developed in EP-A-0,624,126.
Practical ways of carrying out the helical cutting are disclosed in EP-B-0,426,702. This patent also explains how a big angle of melt orientation, even 90xc2x0 to the longitudinal direction of the film, can be achieved by first extruding a tubular film with a xe2x80x9cscrewedxe2x80x9d melt orientation (relative rotation between the extrusion die and the haul-off means) and in a second process helically cutting this tube in a direction which increases the angle of orientation.
Returning to the problem of shock-peel-strength in a heatseal, cross-laminates made according to the method of the above mentioned GB-A-1,526,722 or the above mentioned improvements on this, have since 1978 been used in the manufacture of open-mouth industrial bags sealed in top and/or bottom by a xe2x80x9cpeel-typexe2x80x9d heatseal, and are still used so on a relatively large scale. This includes the use in xe2x80x9cform-fill-and-sealxe2x80x9d in which the conversion of film to bag takes place in conjunction with the filling operation. However, with the existing need for downgauging the film material for bag making, further improvements are strongly needed. It is hereby important to achieve the improvement in heatseal properties without reducing other important strength properties, in particular the yield tension. The latter is important in connection with stacking of the industrial bags filled with powdered or granular material, since the lower layers of bags in a stack often come under a very high static load.
According to the invention claimed in EP-B-0,338,747 which is developed with a special view to industrial bags made from such cross-laminates, the heatseal is protected by an adjacent row of embossments, which is adapted to act shock absorbing and/or force controlling.
Furthermore according to the invention described in WO-A-98/23434 there is provided a heatseal methodxe2x80x94also with a special view to the cross-laminatesxe2x80x94which allows a particular high shrinkage of the polymer film, not only in the bonded zone of the heatseal but also in the immediately adjacent, unbonded zones of the film, at the side of the seal which is or is predetermined to come in contact with the contents of the bag. By this high shrinkage the film grows thick in-the said zones, and this compensates for the loss of orientation in these zones. (As mentioned at the beginning of this description the problems in heatsealing of cross-laminates are caused by the loss of orientation in the unbonded zones adjacent to the bonded zone). The invention involves a pivoting of one of the sealing bars.
However, in spite of the proven positive results of these two inventions, the one to protect the seal by means of embossments and the other concerning the special heatseal method, neither of these has met acceptance in the marketplace. The reason is that both inventions require special machinery installed by the user which convert the cross-laminate to bags and/or fill and close the bags. Consequently there is a strong need to improve the heatseal properties of the cross-laminate itself without negatively affecting the other strength properties, in particular the yield tension.
The cross-laminate according to the present invention is characterised as follows: A webformed cross-laminate consisting of two or more mutually bonded films or film assemblies, each of which is uniaxially oriented or unbalanced biaxially oriented, characterised in that said laminate comprises (A) on one side of the cross-laminate either a) a film in which the main direction of orientation generally follows the longitudinal direction of the web, or b) an assembly of films, the resultant main direction of which generally follows the said direction, and (B) on the other side of the cross-laminate either a) a film in which the main direction of orientation is generally perpendicular to the longitudinal direction of the web, or b) an assembly of films, the resultant main direction of which is generally perpendicular to the longitudinal direction of the web, and in that the coefficient of elasticity (E) of the material A in unoriented state is at least 15% lower than the coefficient of elasticity of the material B in unoriented state and in that upon heating the cross laminate A and B exhibit differential shrinkability along the said longitudinal direction. A being more shrinkable.
The preferred method of manufacturing it is as follows: A method of manufacturing a cross-laminate in which there is formed a sandwich-web of different films, each exhibiting a main direction of orientation, and wherein these directions cross each other, and subsequently the sandwiched films are further oriented below their melting range by being stretched together in the longitudinal direction of the web, and before or after this longitudinal stretching, also in the transverse direction of the web, this transverse stretching being carried out between grooved rollers, and the films in the sandwich are bonded to form a laminate before, during or after said longitudinal and transverse stretching operations, characterised in that said sandwich-web consists of two films or assemblies of films (Axe2x80x2) and (Bxe2x80x2) respectively, (Axe2x80x2) on the one and (Bxe2x80x2) on the other side of the sandwich-web, whereby (Axe2x80x2) is either a) a film in which the main direction of orientation generally follows the longitudinal direction of the sandwich-web, or b) an assembly of films, in which the resultant main direction of orientation generally follows the said direction, and said orientation or resultant orientation is strongerxe2x80x94as measured by shrink testing before (Axe2x80x2) and (Bxe2x80x2) have been brought together to a sandwichxe2x80x94than said orientation or resultant orientation in (Bxe2x80x2) in the same direction, and further (Bxe2x80x2) is either a) a film in which the main direction of orientation is generally perpendicular to the longitudinal direction of the web, or b) an assembly of films, in which the resultant main direction of orientation is generally perpendicular to the longitudinal direction of the web, in which the coefficient of elasticity (E) of (Axe2x80x2) in unoriented state is at least 15% lower than the coefficient of elasticity of (Bxe2x80x2) in unoriented state and said orientation or resultant orientation is strongerxe2x80x94referring to shrink testing before Axe2x80x2 and Bxe2x80x2 have been brought together in a sandwichxe2x80x94than the orientation in (Axe2x80x2) in the said perpendicular direction.
The differences in shrinkability just referred to are preferably at least 10% and more preferably 15%, 20% or even higher.
The coefficient of elasticity (E) of the material from which A is formed in unoriented state is at least 15% lower than the similar property of the material from which B is formed. The objective of this precaution is to make the forces during peeling less concentrated, while making the cross-laminate, seen as a whole, as stiff as possible.
The unoriented state to which E here refers, can be obtained by melting and resolidifying of A after this has been separated from B, and likewise for B. A suitable technique for delaminating A and B is described in more detail below in the example.
When the cross-laminate according to the invention is used to make a bag with a heatseal perpendicular to the main direction of orientation in (A), and the bag filled with particulate product is accidentally dropped, the higher E in the A-material will tend to give the bag highest elongation in the direction perpendicular to the seam, and the shock-absorbtion resulting from the elongation particularly in this direction enhances the impact strength of the seal.
With a view to the geometry of a stack of filled bags and especially the maintenance of a regular geometry of the stack, any extensibility of the bags has negative effect, but in this respect it is the average between the extensibility in the longitudinal and the transverse directions of the bags which counts, and therefore a high extensibility in the main direction of the (A)-material of the cross-laminate can be offset by a correspondingly low extensibility in the main direction of the (B)-material.
Each of the films here referred to may either be co-extruded or be mono-extruded, i.e. they may each consist of several or of only one layer. Furthermore each of the said films may either be truly uniaxially oriented (referring to molecular orientation), or may be biaxially oriented in unbalanced manner, so that in any case they exhibit a main direction of orientation.
The side of the laminates on which the film or group of films designated as A is located is the side predetermined to become bonded by the heatsealing (usually heatsealing to a similar cross-laminate), namely so that the heatseal extends generally perpendicularly to the longitudinal direction of the cross-laminated web. It can be expected that the cross-laminate mainly will be traded in tube form, with a longitudinal overlap seam, which tube may be a gusseted tube, ready for use in form-fill-and seal. In this embodiment the A-surface must be the inner surface of the tube.
Alternatively the cross-laminate may be folded-over without making it a tube, for the purpose of making bags in which the fold forms the bottom and the side-edges are heatsealed. Also in this case the inner surface of the folded-over construction must be the A-surface.
The relatively high shrinkability of A or Axe2x80x2, which is preferably at least 30% and more preferably 35%, 40% or even higher in a direction which is predetermined to become perpendicular to the heatseal, produces a suitably strong contraction and thereby increase of thickness. The heating which inevitably takes place in the neighbourhood of the seal produces differential shrinkage of A and B with a particularly interesting and useful effectxe2x80x94see the photo micrographs FIGS. 8b, c and dxe2x80x94provided the surface regions of the sealing bars are tapered, bent or rounded at the boundaries, as they normally are in order to avoid xe2x80x9cscarsxe2x80x9d in the sealed material.
This differential shrinkage which is based on the claimed properties of the cross-laminate, makes the latter bend in the direction which makes it""s A-side concavexe2x80x94as seen on the photo micrographs at some distance from the bonded zone. However, immediately adjacent the bonded zone where the material is molten and soft, there is compensated for this bending by a bending in the opposite direction so that the A-side here becomes convex. This last bending which indirectly is a result of the differential shrinkage, acts as a kind of peeling in molten state and reinforces the seal. Furthermore, the differential shrinkage has the effect that the laminate is pushed towards the sealing bars also at their tapered, bent or rounded edges, where there is no sealing pressure, and this helps to make the material grow thick in the zone which during shock-peeling is critical.
For the sake of good order it must however be mentioned that the seals shown in the photo micrographs have been made by the special sealing method described in WO-A-98/23434, and thereby the above mentioned effects become further accentuated. (More about the conditions under which the heatsealing was carried out is stated in the example).
A feature stated in the description of the method above is the difference between the shrinkage of A and shrinkage of B, when the two films or assemblies of films have been separated from one another. In fact cross-laminates are usually, with a view to the influence of bonding on tear-propagation resistance, made with a relatively low bonding between the individual films (this has already been mentioned) and therefore such separation will normally be easy even at room temperature.
The separated films may be tested for their shrinkability in any desired direction, for instance in the longitudinal direction with respect to the web, or in the direction perpendicular to that direction, when heated to a predetermined suitable temperature. This temperature is, for instance, close to but below the mechanically determined melting point of A or B, whichever exhibits the lower melting point. In the invention it is preferred that the shrinkability expressed in percentage terms of the A film or film assembly is at least 10 units higher (preferably more) than that of the B film or film assembly, in the longitudinal direction of the web, when the shrinkabilities are measured at the same predetermined temperature which is close to but below the mechanically determined melting point of A or B, whichever exhibits the lower melting temperature. The predetermined temperature may be, for instance, 5xc2x0 C. below the mechanically determined melting point. A convenient measurement temperature when the cross-laminate consists of polyethylene based films is a value in the range 115 to 120xc2x0 C.
In this connection, the mechanically determined melting point is the temperature at which the film or film assembly, as the case may be, most pronouncedly loses its mechanical stability with temperature increase (maximum differential change) and can conveniently be determined directly on the film or (film assembly) by dynamic spectrometry. The melting point may alternatively be determined from the change in tensile properties with temperature.
The shrink properties of Axe2x80x2 and Bxe2x80x2 before cross-xe2x80x9csandwichxe2x80x9d are determined in a similar manner. However if Axe2x80x2 and/or Bxe2x80x2 each consists of more than one film, the determination must take place after bonding of the assembly Axe2x80x3 and/or Bxe2x80x3 without changing the properties, e. g. by means of a soft bicomponent glue.
In the invention advantageously the bonding between A and B can be made weaker than the mutual bonding between the films in the A assembly or the B assemblies when A and/or B consists of more than one film. The advantage of this concerns the general strength properties of the cross-laminate. Suitable values of the bonding strength measured by slow peeling, are between about 0.1 and 0.5 Ncm1, taken as an average between peeling of longitudinally and transversely cut strips. (This indication is not meant to limit the scope of the invention.)
The bonding is preferably facilitated and controlled by coextruded xe2x80x9clamination layersxe2x80x9d on each film used for the cross-laminate, and preferably there are also coextruded xe2x80x9cheatseal layersxe2x80x9d to help two cross-laminates stick together during the heatsealing and/or for other improvements of the surface properties of the laminate.
Normally there will occur some differential shrinkage effect in the cross-laminate of the invention even at ambient temperature, i. e. there will be a tendency to curling around the transverse direction of the web, unless precautions against this are taken. This can be a disadvantage for the handling of the web or of bags made from the cross-laminate. To overcome this problem the cross-section of the web can have a waved shape with stabilized waves, preferably so that the wavelength measured from wave-top to wave-top on one surface of the web is less than 5 mm. However, this waving should preferably be very shallow. In itself it is known to make cross-laminates with such stabilized, shallow waving, see the already mentioned EP A-0, 624,126. The methods here disclosed are directly applicable in connection with the present invention. While such waving of the cross-section of the web totally can eliminate the tendency to curling at ambient temperature, it does not essentially reduce the inventive effects of the differential shrinkage during heat-sealing, since the stabilized waving disappears by transverse shrinkage, when the laminate is heated to temperatures near the melting range.
As it appears from the statement of the method above, the way the sandwiched web is transversely stretched is preferably between grooved rollers, and it is also longitudinally stretched and is bonded to a laminate. There should normally be several steps of grooved rollers stretching, and may also be several steps of longitudinal stretching and of bonding. These processes can be carried out in various sequences, and in this connection the disclosures in the above mentioned GB-A-1,526,722, EP-B-0,276,100 and EP-A-0,624,126 directly apply.
The film or films, which constitute Axe2x80x2, is/are preferablyxe2x80x94prior to the sandwiching with the film or films which constitute Bxe2x80x2xe2x80x94longitudinally oriented by pulling over a frictionally with-holding roller or bar; in other words they are subjected to conventional longitudinal stretching below the melting range, e. g. by being passed over a number of smooth rollers, rotating at different circumferential velocities. It may advantageously be a stretching at temperatures significantly lower than the melting range, e. g. between 20-60xc2x0 C. If spiral cutting of a tubular extruded Axe2x80x2-film is used in making an Axe2x80x2-film, this spiral cutting should take place prior to said longitudinal pulling.
Axe2x80x2 and Bxe2x80x2 may each be one film, optionally a coextruded multi-layer film (see FIG. 1 and the flow sheet FIG. 4) which is simplest, especially when very low gauges of the cross-laminate are wanted. However, with a view to the general strength properties of the cross-laminate, Bxe2x80x2 and/or Axe2x80x2 are/is preferably composed of at least 2 films as shown in FIGS. 2 and 3 and the flow sheets FIGS. 5 and 6. (The angles of +/xe2x88x9260xc2x0 and +/xe2x88x9230xc2x0 indicated in the drawings and in the boxes of the flow sheets are of course examples). Thus, B can be a generally symmetrical arrangement composed of at least 2 films, each with a main direction of orientation forming an angle higher than 50xc2x0 and lower than 90xc2x0 to the longitudinal direction of the web while A can be generally symmetrical arrangement composed of at least 2 films, each with a main direction of orientation forming an angle higher than 0xc2x0 and lower than 35xc2x0 to the longitudinal direction of the web. Axe2x80x2 can be formed of two films which are produced each with a main direction of orientation at an angle higher than zero and lower than 45xc2x0 to its longitudinal direction, either by spiral cutting of a mainly longitudinal oriented tubular film, or by cutting of a mainly longitudinal oriented tubular film, or by twisting a tubular film while it is hauled off from an extrusion die and is in bubble form, the two films subsequently being assembled to form a generally symmetrical sandwich of the two films and this sandwich being subjected to pulling against a frictionally withholding roller or bar.
Instead of using for Axe2x80x2 two films which have been produced by helical cutting under a relatively small angle, it is more practical to use one film with 2 melt-oriented layers formed under use of counter-rotating dieparts as shown in flow sheet FIG. 7. Thus, an Axe2x80x2 assembly can be produced by forming a first cross-laminate of 2 melt-oriented layers by means of counter-rotating dieparts, the lamination taking place immediately before, during or immediately after these layers have left the die, preferably with a minor coextruded layer between which is adapted to reduce the bonding, the melt-orientation in each of the 2 layers forming an angle less than 45xc2x0 to the longitudinal direction of this first cross-laminate, and subsequently subjecting the assembly to pulling against a frictionally withholding roller or bar.
For extrusion of such film and construction of the coextrusion die with counter-rotating dieparts, reference is made to the above mentioned GB-A-1,526,722.